Java 代码

import java.util.Scanner;

public class Main
{
    public static void main(String[] args)
    {
        Scanner cin = new Scanner(System.in);

        String s = cin.nextLine();

        String a = cin.next();
        String b = cin.next();

        System.out.println(s.replaceAll("\\b"+a+"\\b", b));
    }
}